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Hyponatremia

INTRODUCTION — In almost all cases, hyponatremia results from the intake (either oral or intravenous) and subsequent retention of water [1]. A water load will, in normal subjects, be rapidly excreted as the dilutional fall in plasma osmolality suppresses the release of antidiuretic hormone (ADH), thereby allowing the excretion of a dilute urine. The maximum rate of water excretion on a regular diet is over 10 liters per day, thereby providing an enormous range of protection against the development of hyponatremia.

Some patients with primary polydipsia retain water and become hyponatremic because they drink such large quantities of fluid that they overwhelm the excretory capacity of the kidney. In almost all cases, however, hyponatremia occurs because there is an impairment in renal water excretion, due most often to an inability to suppress ADH release.

Although the definition may vary among different clinical laboratories, hyponatremia is commonly defined as a serum sodium concentration ≤135 meq/L [2]. An overview of the causes of hyponatremia will be presented here (table 1). Most of the individual disorders are discussed in detail separately, as are issues related to diagnosis and treatment [1,3]. (See "Diagnosis of hyponatremia" and "Overview of the treatment of hyponatremia".) It should also be emphasized that, in selected patients, multiple factors may contribute to the fall in the plasma sodium concentration. Symptomatic infection with human immunodeficiency virus (HIV) is an example of this phenomenon, as volume depletion, the syndrome of inappropriate ADH secretion, and adrenal insufficiency all may be present. (See "Electrolyte disturbances with HIV infection".)

The presence of hyponatremia, even of relatively mild severity, is associated with adverse survival. This includes patients with heart and/or hepatic failure, and/or acute myocardial infarction. (See "Hyponatremia in cirrhosis" and "Hyponatremia in heart failure".) DISORDERS IN WHICH ADH LEVELS ARE ELEVATED — The two most common causes of hyponatremia are effective circulating volume depletion and the syndrome of inappropriate ADH secretion, disorders in which ADH secretion is not suppressed [4]. Effective circulating volume depletion — Significantly decreased tissue perfusion is a potent stimulus to ADH release. This response is mediated by baroreceptors in the carotid sinus, which sense a reduction in pressure or stretch, and can overcome the inhibitory effect of hyponatremia on ADH secretion. Thus, hyponatremia can develop in patients with any of the following disorders. True volume depletion — True volume depletion from gastrointestinal or urinary losses or bleeding will increase ADH and result in hyponatremia if there is adequate water intake. Such patients may also have hypokalemia and azotemia due to decreased renal perfusion. This constellation of findings in patients with a large villous adenoma has been called the McKittrick- Wheelock syndrome.

The replacement of severe diarrheal losses due to cholera (which is associated with a sodium concentration in stool of 120 to 140 meq/L) with an oral rehydration solution with reduced osmolality may result in an increased incidence of hyponatremia as compared to replacement with standard (and higher sodium concentration) oral rehydration therapy [5]. (See "Oral rehydration therapy".)

Exercise-associated hyponatremia — Marathon and ultramarathon runners can develop potentially severe hyponatremia that is primarily due to excessive water intake combined, in many cases, impaired water excretion due to persistent ADH secretion. A similar sequence can occur during military operations and desert hikes. (See "Exercise-associated hyponatremia".) Heart failure and cirrhosis — Even though the plasma volume may be markedly increased in these disorders, the pressure sensed at the carotid sinus baroreceptors is reduced due to the fall in cardiac output in heart failure and to peripheral vasodilatation in cirrhosis...

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Hyponatremia in the older adult.
Management of defects in water homeostasis in the elderly is often difficult because of age related changes and diseases that are associated with impairment of water metabolism. The feeling of thirst is often impaired in the elderly (Kugler, 2000). Hyponatremia is a serum sodium concentration of less than the normal 137 mmol per litre (Farrell, 2007). This essay will explore why this condition happens to the older adult (over 65 years) and how to assess for it. The effects of this condition on the elderly will be explored. Problems that make it difficult for nursing assessment will be identified and recommendations will be made on how to overcome these barriers.
Sodium is the most abundant electrolyte in the extracellular fluid. It controls water distribution through the body and a loss of sodium is usually accompanied by a loss of water (Farrell, 2007). The normal range for serum sodium level is 135 to 145 mEq/l (Bruck, 2005). Sodium in the body is determined by how much salt is in the diet and how the intestines absorb it (Bruck, 2005). Sodium helps to maintain normal blood pressure, supports the work of the nerves, muscles, and regulates the body’s fluid balance. When the sodium level in the body becomes too low, extra water enters...